1. Field of the Invention
This invention is related in general to carburetors for internal-combustion engines that comprise a feedback control system responsive to the composition of the engine exhaust gases. In particular, this invention provides a new device for improving the air-fuel mixture supplied to the engine by controlling the pressure in the float chamber of the carburetor.
2. Description of the Related Art
As is well understood in the art, conventional internal-combustion engines are fueled with an air-fuel mixture that is formed in the carburetor and passed to the intake manifold of the engine. Referring to the schematic representation of FIG. 1, ambient air is drawn by the engine suction into the intake manifold 2 through a venturi tube 4 contained within the body of the carburetor 6. The flow of air is controlled by the position of a throttle 8, which normally consists of a butterfly valve operated by a user by means of a remote linkage system. When the valve is closed and the engine is idling, little air passes through the venturi tube, so that little or no fuel is drawn into the air stream by the venturi effect in tube 4; the fuel is instead drawn by the engine's suction directly from the float chamber or bowl 10 into the manifold 2 through an idle bypass circuit 12. As the throttle valve is opened and more air is allowed to pass through the venturi tube, the decrease in static pressure created by the restriction in the tube causes a pressure differential that results in fuel being delivered to the air stream within the venturi tube itself through a main jet system 14. As the throttle is further opened and the engine's speed (rpm) increases, more air is drawn causing a yet lower static pressure within the venturi tube and greater fuel flow rate into the air stream.
In order to optimize fuel efficiency and pollution control, the air-fuel ratio in the mixture flowing to the engine should at all times be equal to the stoichiometric ratio required for full combustion. This is impossible to achieve with a system that relies on a number of fixed-size jets to meter the fuel flow to the intake manifold. Therefore, in designing a carburetor, the dimensions of the jets in the idle bypass circuit 12 and in the main jet system 14 are chosen to provide air-fuel ratios corresponding to optimal overall performance within the range of operation of the engine. Typically, the mixture is richer than the stoichiometric requirement (that is, it contains more fuel than necessary for complete combustion) at idle speeds and it becomes progressively leaner at higher speeds. The resulting effect is that the air-fuel ratio is sub-optimal nearly at all times. Thus, additional methods of controlling the air-fuel ratio are required for optimal performance.
From the foregoing and from basic principles of fluid dynamics it becomes apparent that the pressure in the float chamber of a conventional internal-combustion engine carburetor affects the air-fuel mixture delivered to the engine. In conventional carburetors, the float chamber is kept at substantially atmospheric pressure by means of a vent typically connecting the chamber to a region downstream of the air filter. As a result, the air-fuel ratio is determined only by the pressure in the venturi tube (or manifold, at idle speed) and by the metering of the various jets in the carburetor as fuel is drawn from the float chamber by the suction created in the main venturi tube. By varying the pressure in the float chamber, an additional control variable is available that can be used to regulate the air-fuel ratio to the engine. Several patents have described devices that utilize this principle in a feedback control loop system for optimizing the composition of the air-fuel mixture at all times during operation. Typically, these systems measure the oxygen content in the engine's exhaust and utilize it as a measure of the deviation of the air-fuel ratio from the optimal mixture. This information is then used to generate a control signal for varying the pressure in the float chamber. If the exhaust's oxygen content indicates that the mixture is too rich, the pressure is decreased, resulting in a reduced flow rate of fuel to the venturi tube and, accordingly, a leaner mixture. The opposite control action is produced, of course, when the mixture is too lean.
For example, U.S. Pat. No. 3,742,924 issued to Bachle (1973) describes a device for providing variable ambient pressure in the float chamber of a carburetor. The pressure variations are produced by a valve installed in a tube connecting the chamber to the venturi of the carburetor, so that a vacuum (and a leaner mixture) is obtained when the valve is open. The control of the valve is effected by a solenoid driven by the signal generated by a sensor in the exhaust pipe of the engine.
In U.S. Pat. No. 4,034,727 (1977), Aono et al. describe a similar device where the pressure variation in the float chamber is produced by a vibrating diaphragm built into the vapor side of the chamber. The diaphragm is driven by an electromagnetic transducer, which is itself controlled by a signal designed to optimize the fuel mixture under varying operating conditions.
U.S. Pat. No. 4,034,730 to Ayres et al. (1977) discloses a carburetor where the pressure of the fuel in the float chamber is determined by the operation of an electric fuel pump. The pump in turn is controlled by electronic circuitry responsive to a sensor in the exhaust pipe of the system. When the fuel mixture is too lean with respect to a set point for the driving conditions, the pump produces a higher pressure and more fuel is supplied to the venturi. The converse occurs, on the other hand, when the mixture is too rich.
U.S. Pat. No. 4,191,149 to Dutta et al. (1980) shows a carburetor where the pressure in the float chamber is varied by means of a line connected to the restriction of a venturi tube. The tube is coupled to a compressor on one side and to a valve open to the atmosphere on the other, so that the pressure drop across the venturi is affected by the opening of the valve. As the valve is closed, the pressure in the venturi increases, also causing the pressure in the float chamber to increase and produce a richer mixture. A system of orifices in every segment of the system is used to optimize the effect of the valve on the float chamber pressure. In another embodiment of the invention, air is drawn by the vacuum in the exhaust manifold from the outside atmosphere into a venturi tube connected to the float chamber. The air flow is regulated by a valve actuated by a controller responsive to the signal generated by an oxygen sensor in the exhaust stream. When the valve is closed, a vacuum is transmitted to the float chamber; as the valve opens, air is drawn from the outside through the Pitot tube and the pressure in the float chamber increases accordingly.
In U.S. Pat. No. 4,512,304 (1985), Snyder describes a device for regulating the supply of fuel to the engine. Pressure is applied to the regulator to cause a predetermined rate of flow of gasoline to the carburetor, thereby affecting the air to fuel ratio. The pressure exerted is a result of a signal from an exhaust gas sensor.
Finally, U.S. Pat. No. 4,363,209 to Atago et al. (1982) discloses a carburetor system that produces a negative pressure on the fuel jets in order to vary the throughput to the venturi. The negative pressure is obtained by connecting a vacuum source to the jet ducts by means of a valve responsive to a control circuit connected to an exhaust gas sensor.
All of these systems require either modifications to the design of a conventional carburetor or additions of expensive apparatus to standard equipment. Therefore, they are not economically suitable for after-market application. In addition, physical constraints often limit the ability of some devices to perform according to their design specifications. For example, we found that the second embodiment of the invention described in the Dutta et al. patent is not practically feasible for correcting a lean mixture because a very large air intake would be required to generate a positive pressure to the float chamber through a Pitot tube. This air would then flow to the intake manifold and further dilute an already lean mixture, thus aggravating the condition and providing no effective control.
Therefore, there still exists a need for simpler and more effective system for optimizing an engine's air-fuel ratio by varying the pressure in the float chamber of the carburetor. The present invention is directed at apparatus that permits the easy and relatively inexpensive conversion of a conventional carburetor to a feedback-controlled system that effectively varies the air-fuel ratio for optimal operation under all conditions.